People lay red carnations on pictures of Armenian intellectuals, detained and deported in 1915, during a rally to commemorate the 101th anniversary of the 1915 mass killing of Armenians in the Ottoman Empire.

The Anti-Defamation League said Friday that the massacre of 1.5 million Armenians that began in 1915 was “unequivocally genocide” and for the first time expressed support for US government recognition of the killings as a genocide.

The statement from ADL chief executive officer Jonathan Greenblatt was posted on the organization’s website. It marked the strongest affirmation yet from the leading civil rights group in support of Armenian advocates, who have long pushed for formal recognition of what they contend was a genocide perpetrated by Turkish forces.

“What happened to the Armenian people was unequivocally genocide,” Greenblatt wrote. “We believe that remembering and educating about any genocide — Armenian, the Holocaust, Bosnia, Rwanda, and others — is a necessary tool to prevent future tragedies. . . . That is why I am speaking out today and why we would support US recognition of the Armenian Genocide.”

The statement came after years of tension between the ADL and Armenian activists, who chafed at the organization’s prior reluctance to acknowledge the killings as genocide.

Anthony Barsamian, the Boston-based cochairman of the Armenian Assembly of America, welcomed Greenblatt’s statement and said he hoped the ADL would back a resolution pending in Congress.View StoryCardinal O’Malley leads prayer service for Armenian genocide

Nearly 800 people attended a first-ever prayer service organized by the Archdiocese of Boston to commemorate the Armenian genocide.

The measure calls on President Obama to work for “Armenian-Turkish relations based upon the Republic of Turkey’s full acknowledgment of the facts and ongoing consequences of the Armenian Genocide, and a fair, just, and comprehensive international resolution of this crime against humanity.”

When World War I broke out, Turkish leaders targeted the Christian Armenian population under the pretext that they would cooperate with the Russian enemy. Turkey has long denied that a genocide took place, saying the death toll is exaggerated and that many Armenians died of from starvation or disease amid the upheaval of the war. The Turkish embassy in Washington could not be reached

Aram Hamparian, executive director of the Armenian National Committee of America, said Greenblatt’s statement “is new in that it’s very explicit in breaking with [the Turkish government’s] denials.”

While the ADL has never denied that a massacre took place, the organization said in 2007 that a similar congressional push was “counterproductive.”

The following year, the ADL’s then-national director, Abraham H. Foxman,said in a letter to an official in Watertown, which has a large Armenian population, that “we have referred to those massacres and atrocities as genocide.” Foxman also made explicit reference to the “Armenian genocide” during a speech in 2014.

On Friday, Robert Trestan, the ADL’s New England regional director, said Greenblatt’s statement was the “most unequivocal statement that we’ve ever issued,” and that the ADL and local Armenian community share an interest in educating the public about genocides to prevent future atrocities.

But Andrew H. Tarsy, former regional director of the ADL who split with the group over the issue, said Greenblatt’s statement was inadequate.

“I think they ought to lead the conversation about reparations for these families,” he said. “The recovery of assets, land, money, items, family heirlooms. Everything that Holocaust reparations . . . has represented should be on the table.”

In a statement, the US State Department said the government “clearly acknowledges as historical fact and mourns that 1.5 million Armenians were massacred or marched to their deaths in the final days of the Ottoman Empire. . . . The United States continues to urge [Turkey and Armenia] to work together to achieve a full, frank, and just acknowledgment of the facts. ”

Friday’s declaration by the ADL was also praised by Middlesex Sheriff Peter Koutoujian, a prominent member of the local Armenian community.

“Today’s historic statement was the result of a lengthy, open dialogue I was proud to be part of, along with members of the Armenian National Committee of America, the Armenian Assembly of America, as well as individuals of the Armenian American and Jewish communities of Greater Boston,” Koutoujian said. “And so I want to personally thank all those who took part in these discussions, especially ADL New England Regional Director Robert Trestan who has been a true friend and partner to all Armenians.”